I investigated cellular mechanisms of ion and acid-base transport in rainbow trout (Oncorhyncus mykiss), crabs (Neohelice granulata), zebrafish (Danio rerio), Pacific hagfish (Eptatretus stoutii), and mosquito larvae (Aedes aegypti) with a primary focus on discerning the mechanisms governing ion transport and acid base regulation. In rainbow trout I provide the first functional evidence for two physiologically distinct mitochondrion-rich (MR) cells at the gill and demonstrate a new model for transepithelial Na+ uptake from freshwater involving apical Na+ channels and basolateral Na+/HCO3- co-transporters. These data are supported by extensive thermodynamic consideration of Na+ uptake from freshwater. I also demonstrate functional Cl-/HCO3- exchangers in both MR cell subtypes with roles for Cl- uptake and intracellular pH (pHi) regulation respectively and I present the first evidence for a Cl- dependent Na+/H+ exchanger in gill MR cells. Finally I demonstrate a unique Na+ dependent pHi recovery mechanism that requires protein kinase C for activation. A major limiting factor in clarifying the mechanisms of Na+ uptake in freshwater fish is the lack of a typical Na+ channel in any of the fish molecular databases. My work on zebrafish, although preliminary, indicates that a member of the acid-sensing ion channel family could be responsible for Na+ uptake from freshwater. I then expanded my research outside the trout model using an isolated crab gill preparation. I provide a cellular model for H+ secretion in crab gills that supports the transepithelial Na+ transport model that I described in rainbow trout. In Pacific hagfish, I demonstrate that recovery from blood acidosis is dependent on a Na+/H+ exchanger in gill MR cells. This mechanism of regulation involves translocation from the cytoplasm to the apical membrane during acidotic stress. This data combines with other studies demonstrating the mechanisms of acid and base secretion from a single MR cell subtype. Finally, I show that serotonin stimulation alkalinizes the pHi of the anterior midgut cells in the larval mosquito to levels never before observed in cell biology. These data challenge the dogma of pHi regulation in cell biology and demonstrate the power of using a comparative approach to systems physiology. / Physiology, Cell and Developmental Biology

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